Already under fire from one Lantus challenger, Sanofi is suing Mylan in an effort to hold off that company's version of its top-selling drug. Sanofi says Mylan is infringing 18 patents and asked a court to intervene in the generic drugmaker's launch efforts.

In a 70-page lawsuit filed on Tuesday in federal court in New Jersey, Sanofi says it turned to the legal process after efforts to get more info on Mylan's program in out-of-court negotiations proved "unsuccessful."

Mylan is far from the only threat to Sanofi's top moneymaker, a drug that reeled in €5.7 billion ($6.7 billion) around the world last year. In addition to competitive pressures in diabetes that have taken a bite out of pricing power, the company also faces several copycat attacks.

Under a 2015 settlement, Eli Lilly has already launched Basaglar. The medication, which hit the market late last year, has generated $278 million so far in 2017, Lilly reported with its third-quarter earnings on Tuesday. But because there's only one marketed Lantus rival so far, discounting to the brand has remained limited. If a third or a fourth player were to enter the fray, revenues could increasingly deteriorate for Sanofi.

Merck has its own approved version of Lantus but can't launch it until 2019 due to a 30-month stay under the Hatch-Waxman Act that started ticking in September 2016. Alternatively, the company could launch earlier if it defeats Sanofi in patent court. Mylan is also challenging Sanofi's Lantus patents, according to the latter drugmaker's Tuesday release on the lawsuit.

Mylan is working on its version of Lantus with Biocon and filed for FDA approval last month, according to Sanofi's lawsuit. Also last month, Biocon's insulin plant in Malaysia won approval from the EU, where the partners are also seeking approval of their drug. Biocon already has an approval in Japan for its Lantus copycat.

According to the FDA's Orange Book, patent expirations on Sanofi's Lantus and Lantus SoloStar range from 2023 to 2028. Mylan didn't immediately provide a response to the lawsuit.